Photochromism, as applied to the present invention, define a property of a light-transmitting article whereby, upon stimulation by actinic radiation, the light-transmitting capability of such article is substantially reduced. Upon removal or diminution of the actinic radiation, the light-transmitting capability of the article is increased. Such reduction and return of the light-transmitting capability of articles contemplated by the present invention may be repeated through numerous cycles.
Photochromic light-transmitting articles in the form, for example, of lenses which may be employed in ophthalmic applications, have been known for many years and are now commercially available. Such commercially available lenses comprise silver halide materials dispersed in a glass matrix which involves rather complicated, high-temperature techniques and in situ formation of the silver halide materials in the matrix. See, for example, U.S. Pat. No. 3,208,860. According to the disclosure of that patent, silver halide crystals are dispersed in inorganic silicate glass and upon the application of radiation actinic to such a system, the silver halide materials react in such a way as to provide a diminution in the transmission characteristics of the glass. Upon removal of the actinic radiation, the silver halide crystals apparently return to their original transparent state and, accordingly, the transmission characteristics of the glass improve. Such systems basically comprise microscopic areas of entrapment of silver halides within the glass matrix -- the crystal size being sufficiently small as to minimize light-scattering. The preferred method disclosed for making such photochromic articles is to incorporate the constituents of the desired crystalline phase in molten glass and thereafter precipitate the crystals in situ in the glass matrix. Heat treatment of the ultimately formed material is recommended to allow rearrangement of silver cations and halide anions and thereby form a separate phase within the glass matrix which assures appropriate photochromism. It is stated that such heat treatment comprises exposure of the glass article to the strain point temperature for about 16 hours, or to the softening temperature for about 15 minutes. U.S. Pats. Nos. 3,293,052 and 3,449,103 assigned to the same assignee as the aforementioned U.S. Pat. No. 3,208,860, relate to improvements in the basic photochromic glass system.
U.S. Pat. No. 3,466,216 describes a photochromic system wherein silicates comprising entrained silver halide materials are dispersed in a plastic binder and cast as a film. For reasons described in the above noted patents, such films may be made by taking a photochromic glass such as defined in the above-noted patents, grinding it and dispersing the ground glass in a suitable plastic binder. Another defined method is the production of tiny glass beads comprising silver halide crystals which may be dispersed in a binder and cast into a film.
In an article by A. F. Pervey and A. V. Milahaylov entitled AGCL - CUCL PHOTOCHROMIC COATINGS, "Soviet Journal of Optical Technology" February, 1972, photochromic films were produced by the simultaneous vacuum evaporation of silver chloride and cuprous chloride. Photochromism of the film was observed but, in accordance with the disclosure of copending U.S. patent application Ser. No. 672,237, it must be appreciated that the photochromic effect of such a system would probably not be continuously reversible since no method is disclosed for preventing loss of the reactants which are required to provide the requisite photochromic effect. Note, for example, the curves shown in the Perveyev et al. article.
Light transmitting articles having photochromic properties have been prepared which comprise a thin, uniform layer of silver halide sandwiched between, and in intimate contact with, continuous layers of materials which are substantially impermeable to the halogen comprising the silver halide. Such photochromic articles are disclosed and claimed in copending U.S. patent application Ser. No. 672,237. They show remarkable photochromic behavior but do exhibit substantial haze when viewed against a dark background while being illuminated. This is apparently due to light scattering from relatively large particles of silver. This haze accompanying the photochromic effect detracts from the transparency and appearance of the photochromic article.